This invention relates particularly to audiometer systems, for use in determining the hearing capability of test subjects. Conventional clinical audiometers are commonly constructed in accordance with the basic principles of the von Bekesy U.S. Pat. No. 2,563,384. The von Bekesy system comprises a variable frequency acoustical generator, the output of which is connected to a variable resistance network. The variable resistance network conventionally includes a contact slide which is positioned along the resistance network by means of a reversibly driven electric motor. The resistance network desirably has a logarithmically variable output as a function of linear variation in the position of the contact slide.
In the conventional von Bekesy type audiometer system, an input signal is generated at a predetermined frequency, and the test subject actuates the drive motor for the contact slide, causing the power level of the applied signal to be controllably varied. Initially, the applied signal is subject to maximum power attenuation by the resistor network. However, by appropriate actuation of the drive motor, the slide is caused to move along the network, progressively reducing the signal attenuation, until the subject is first able to hear an audible sound, typically applied through a set of headphones. After hearing the first sound the subject actuates a switch to reverse the slide-positioning motor. This causes progressively increasing attenuation of the signal, until no sound is heard, at which time the subject again reverses the slide-positioning motor. This process of motor reversal is continued for a short period of time, permitting a rather accurate observation to be made of the subject's hearing threshold at a given frequency.
Usually, the contact slide for the resistance network is coupled with a tracing pen and a moving chart, so that a continuous graph is drawn of the movements of the contact slide. By proper calibration of the chart, an accurate graph, reflecting the hearing capability of the subject, is automatically prepared, as the subject successively reverses the slide positioning motor.
The von Bekesy type purpose system has proven to be effective for its intended purposes and has been widely adopted for clinical testing. However, it suffers the disadvantage that the electro-mechanically variable atttenuation system, comprising a motor driven contact slide and related elements, is a very expensive unit. It must be manufactured with great precision and must be carefully maintained; it represents a significant cost factor in the complete audiometer. Although it is well known, as a generality, that electronic means frequently can be substituted for electro-mechanical systems, it has nevertheless been considered impracticable to provide electronic attenuation in an audiometer, because of the extreme wide range of power output required (typically 100 decibles) and the rather stringent requirements regarding the purity of the output at the lower power levels. Thus, at the lowest levels of output, an audiometer may require a signal voltage on the order of a fraction of a micro-volt. However, even in the highest quality solid state circuits, the electronic "noise" may be as much as a micro-volt, so that the low level signals would be masked by the noise. Inasmuch as clinical audiometers are required to be effectively free of noise, a condition cannot be tolerated in which the noise represents even a substantial fraction of the signal voltage.
The present invention provides a unique and advantageous electronic attenuator system which is entirely suitable for use in clinical audiometers and which is substantially less expensive and more reliable and durable than conventional electro-mechanical attenuators.
In accordance with the present invention, a von Bekesy type audiometer is provided, in which the conventional electro-mechanical attentuator is replaced by a novel system of selectively operable resistance combinations, actuated in predetermined sequence by a source of timed pulses, synchronized with a recording device, in conjunction with an up/down decade counter system actuated by the pulse source. When the attenuator system is in operation, the attenuation of a predetermined input signal is controlled by successively connecting into circuit a series of control resistors to effect incremental changes in the voltage level of the applied signal. By appropriate selection of resistor values, the incremental changes in voltage may be controlled to suit the specific application. In the case of a clinical audiometer, incremental changes of one decibel are desirable. To accomplish this, the circuit of the invention includes a novel arrangement of attenuator control resistors effective in increments of ten decibels and increments of one decibel, enabling the entire power range to be covered in units of one decibel.
Because of the enormous power range required in a clinical audiometer (0-100 decibels, typically) circuit requirements in the lower end of the power range are significantly different from the circuit requirements in the higher end of the power range. To accommodate effectively these different requirements, the system of the invention provides for the supply of an input signal, at an effective power level in mid-region of the overall power range but at a voltage level at the maximum level required at the system output. This input signal, controllably attenuated in accordance with a predetermined sequence, is applied to a high ratio voltage divider formed in part by the system output, typically a set of headphones. By providing for the signal applied at the voltage divider to be a large multiple of the signal applied to the headphones, it is possible to effectively mask the electronic noise which is inherent in the solid state electronic sequencing system employed in conjunction with the attenuator control resistors.
In accordance with an important aspect of the invention, novel circuit arrangements are provided for achieving power output levels in the higher ranges, extending from the predetermined mid-range level to the level of the desired maximum power output. To this end means are provided for effectively eliminating from the output circuit the high ratio voltage divider, so that the maximum effective voltage is made available at the output stage of the system. Moreover, since the current requirements at the higher power levels are enormously greater than in the lower power ranges, the novel circuitry of the system includes means for amplifying the input current and thereby supplying the necessary output current requirements without overloading or unbalancing the attenuator system.
In a specific embodiment of the invention intended especially for clinical audiometers, the input signal is applied to the voltage divider circuit at a maximum level (assuming no attenuation) sufficient to provide the equivalent of a 60 decibel output at the headphones. The new circuit arrangements include means operative, when actuated, to effectively eliminate the high ratio voltage divider, making available an increased signal voltage, 40 decibels greater, providing a maximum effective output voltage level of about 100 decibels. An appropriate current amplification is also introduced at this stage. Progressively varied attenuation of the input signal level is effected through primary and secondary voltage attenuating resistance networks. The primary network includes a main resistor, which is selectively connected to ground through any one of five attenuator control resistances, forming voltage divider combinations corresponding to voltage level attenuation of -50 to -10 decibels. The secondary network also includes a main resistor which is selectively connected to ground through any of a series of resistors, forming voltage divider combinations corresponding to attenuation levels of -10 to -1 decibels. By appropriate selection of control resistances in these primary and secondary networks, any level of attenuation may be achieved from 0 to -60 decibels, in increments of one decibel.
In the power output range from 99 decibels to 60 decibels, incremental attenuation is achieved by appropriate selection of control resistances in the two networks, in conjunction with actuation of an amplifier circuit which effectively eliminates the high ratio voltage divider and thus effectively makes available 40 decibels of voltage to the output. Thus, activation of the amplifier system in conjunction with variable attenuation of the incoming signal of up to -40 decibels from its maximum will provide the necessary incremental attenuation in the 60-99 decibel output range.
Because of the enormous increase in power requirements in the upper power ranges, the amplifier circuit which becomes effective at the 60 decibel level, includes a current amplifier operative to supply the additional current flow. Significantly, the operation of the amplifier circuit is such that the amplifier cannot, under any circumstances, self bias to a conductive state. Accordingly, when the system is operated in the lower power ranges, the amplifier system does not contribute electronic noise to the system output.
For a better understanding of the above and other features and advantages of the invention, reference should be made to the following detailed specification and to the accompanying drawing.